Our initial investigations (Proc. Soc. Natl. Acad. Sci. U.S. 75, 5864 (1978)) suggest that fibronectin on the platelet surface is the receptor for collagen. One of the principal pieces of evidence was that monospecific antibodies against human fibronectin were capable of stimulating platelets, whereas, preimmune IgG was not. Since then we have shown that F(ab')2 fragments of purified goat anti-human fibronectin antibodies were also capable of stimulating human platelets but that the Fab' fragments were not. However, preincubation of human platelets with immune Fab' fragments inhibited the platelet:collagen interaction. Our current investigations are being conducted to determine the location of platelet fibronectin and to determine whether the platelets' own fibronectin is on the membrane or whether plasma fibronectin remains bound to the platelet membrane after removal of plasma components.